1. Field of the Invention
The present invention relates to a fuel cell system, and more particularly, to a fuel cell system having an improved stack structure.
2. Discussion of Related Art
A fuel cell directly transforms chemical reaction energy obtained by reacting oxygen with hydrogen contained in a hydro-carbonaceous material such as methanol, ethanol, natural gas, etc., into electric energy. Fuel cells can be classified as a high temperature fuel cell or a low temperature fuel cell.
Here, the low temperature fuel cell may include a polymer electrolyte membrane fuel cell (PEMFC), a direct liquid feed fuel cell (DLFC), etc. A DLFC employing methanol as fuel is called a direct methanol fuel cell (DMFC).
Among these fuel cells, the PEMFC has advantages as compared with other fuel cells in that its output performance is excellent; operation temperature is low; and start and response are quickly performed. Therefore, the PEMFC can be widely used as a portable power source for an automobile, a distributed power source for a house and public places, a micro power source for electronic devices, etc.
The PEMFC includes a stack, a reformer, a fuel tank, a fuel pump, an air pump, etc. The stack is formed by an electricity generating assembly including a plurality of unit cells, and the fuel pump supplies fuel from the fuel tank to the reformer. The reformer reforms the fuel to generate hydrogen gas, and supplies the hydrogen gas to the stack. Then, the stack causes the hydrogen gas to electrochemically react with the oxygen gas of air, thereby generating electrical energy.
The DMFC has the same structure as the PEMFC, but directly employs liquid methanol instead of the hydrogen gas as a reaction fuel. The DMFC can thus be manufactured having a small size because peripheral devices such as the reformer are not needed. Storage and treatment of fuel is simplified, and the DMFC can be applied to nonpolluting vehicles, residential power systems, mobile communication devices, medical devices, military equipment, aerospace industrial equipment, etc., because it operates at normal temperatures.
In such a fuel cell system, the stack substantially generating electricity has a structure including several or dozens of unit cells which each include a membrane electrode assembly (MEA) and a separator. The separator is generally called a bipolar plate by those skilled in the art. The MEA has a structure that an anode and a cathode are attached leaving an electrolyte membrane therebetween. Further, the separator is placed in opposite sides of the MEA, and employed as not only a passage for supplying fuel gas and the oxygen gas needed for reaction of the fuel cell, but also a conductor to electrically connect the anode and the cathode of each MEA in series.
Therefore, the separator allows the fuel gas containing hydrogen to be supplied to the anode while the oxygen gas containing oxygen is supplied to the cathode.
Therefore, the fuel gas containing hydrogen and the oxygen gas containing oxygen are supplied to the anode and the cathode through the separator, respectively. In this process, the fuel gas is electrochemically oxidized in the anode, and the oxygen gas is electrochemically reduced in the cathode, so that electrical energy is generated by the movement of electrons, concomitantly generating heat and water.
The conventional fuel cell allowing the electrons to be movable has been disclosed in Japanese Patent Publication No. 2000-164234. In this fuel cell, the unit cells are connected using separate terminals so that the electric energy is generated having predetermined electric potential.
However, in the conventional fuel cell, the terminals protrude from opposite sides of the stack, so that the stack becomes bulky. Further, wiring connecting the terminals causes the size of the fuel cell to become large.
Thus, the foregoing problems discourage the application of the fuel cell to small devices such as a notebook computer, a portable digital video disc (DVD) player, a personal digital assistant (PDA), a cellular phone, a camcorder, etc.